Light emitting element and method of fabrication thereof

ABSTRACT

This invention provides a light-emitting element that comprises a light-emitting portion made of a nitride semiconductor; and a first wavefront converter for converting the radiated shape of light that is emitted from the light-emitting portion into a radiated shape that is smaller than the wavelength thereof, and emitting the same as output light. In this case, the first wavefront converter has a small aperture of a diameter that is smaller than the wavelength of light that is emitted from the light-emitting portion. If the output light is made to comprise an evanescent wave that is emitted to the exterior through this small aperture, it is possible to obtain an extremely small light spot. This invention also relates to a surface-emitting type of light-emitting element comprising a multi-layered structure comprising a light-emitting layer; and a pair of electrodes for supplying a current to the light-emitting layer; wherein output light is emitted from a light-emitting surface on top of the multi-layered structure; and the pair of electrodes are recessed from the light-emitting surface to the light-emitting layer side. This makes it possible to bring the light-emitting surface extremely close to an object to be illuminated. The small aperture can be opened up in a self-aligning manner by using the light from the light-emitting portion. As a result, it is possible to provide a light-emitting element and a method of fabrication thereof that create beam characteristics that are suitable for use with an optical disc or the like.

BACKGROUND OF THE INVENTION

The present invention relates to a light-emitting element and a methodof fabrication thereof using nitride semiconductors and, in particular,to a light-emitting element and a method of fabrication thereof thatmake it possible to extract light of an extremely small spot size.

It has recently become known to use nitride semiconductors such as GaNas materials for light-emitting diodes and semiconductor lasers in thewavelength region from blue to ultraviolet. These materials areattracting attention because they have direct-transition band structuresand can achieve high light-emitting efficiencies. In particular,research and development is proceeding on semiconductor lasers usingnitride semiconductors, which emit light of an extremely shortwavelength on the order of 400 nm, so they can be expected to act aslight sources for reading and writing data with respect to high-densityoptical discs having a capacity of at least 15 gigabytes per side.

Note that the term “nitride semiconductors” as used in this documentcomprises semiconductors of all compositions given by the chemicalformula In_(x)Al_(y)Ga_(z)N (where x≦1, y≦1, z≦1, and x+y+z=1), whereeach of x, y, and z is varied throughout its respective range. Forexample, InGaN (where x=0.4, y=0, and z=0.6) is comprised within theterm “nitride semiconductors.” Furthermore, semiconductors wherein partof the indium, aluminum, or gallium, which are elements of the groupIII, is replaced with boron (B) or part of the nitrogen, which is anelement of the group V, is replaced with arsenic (As) or phosphorous (P)are also comprised therein. In this case, semiconductors comprise anyone of the three elements (In, Al, and Ga) listed above as group IIIelements and always comprise the nitrogen (N) as a group V element.

In such a semiconductor laser, the light emitted from the lasing region,in other words, the light-emitting portion thereof, expands rapidly withdistance therefrom. When such a laser is used as a light source of anoptical disc system, the light must be focused with a lens.

However, there are problems in that it is difficult to design thediameter and curvature of the lens because the lasing spot of ashort-wavelength semiconductor laser is generally small and the lasingwavelength is so short at approximately 400 nm, and it is also extremelydifficult to align the optical axes of the laser and the lens.

In addition, the diameter of the spot at the diffraction limit that canbe focused by a lens is proportional to the wavelength, so thatshortening the wavelength of the light source is an important techniquein increasing the recording density. However, simply shortening thewavelength makes it impossible to focus the projected light to a tinyspot. In other words, it is necessary to develop some sort oflateral-mode control structure in order to obtain suitable beamcharacteristics. In general, crystalline growth and machining techniqueshave not yet been developed far enough for nitride semiconductors,however, and thus there is a problem in that it is difficult toimplement a complicated lateral-mode control structure. In other words,there are still many problems to solve in the implementation of beamcharacteristics that can be used with an optical disc with a system thatcan achieve continuous room-temperature lasing using an InGaAlN laser.

If the wavelength of the light is short, moreover, precision control isrequired for the accuracy and adjustment of the optical system that isused. To reduce the wavelength and spot diameter in this case,aberration and other problems of the lens must also be controlled tosmall values. Thus problems increase as the wavelength shortens, makingit difficult to implement and adjust such a high-precision opticalsystem.

As discussed above, it is difficult to fabricate a lateral-mode controlstructure with a nitride semiconductor laser and it is difficult toimplement beam characteristics that can be used for optical discs. Whensuch a short-wavelength light-emitting element is used as a light sourcefor an optical disc or the like, it is difficult to implement ahigh-precision optical system that is matched to the wavelength, and toadjust such an optical system with a high degree of precision.

SUMMARY OF THE INVENTION

The present invention was devised in the light of the above describedproblems and has as an objective thereof the provision of alight-emitting element that is provided with beam characteristics thatmake it suitable for use with an optical disc or the like.

The gist of this invention makes it possible to implement beamcharacteristics that are suitable for use in an optical disc system orthe like, by the provision of a wavefront converter in ashort-wavelength light-emitting element.

In other words, the light-emitting element of the present inventioncomprises a light-emitting portion made of a nitride semiconductor; anda first wavefront converter converting the radiating range of light thatis emitted from the light-emitting portion into a radiating range thatis smaller than the wavelength thereof, and outputting the same asoutput light.

In this case, if the first wavefront converter has a small aperture suchas a pinhole that has a diameter that is smaller than the wavelength ofthe light that is emitted from the light-emitting portion, and theoutput light comprises an evanescent wave that is output to the exteriorthrough this small aperture, it is possible to obtain an extremely smalllight spot.

The light-emitting element could be further provided with a transparentdielectric layer on the light-emitting surface of the small aperture.

If the light-emitting element is further provided with a transparentprotective film that is coated onto an inner wall of the small aperture,it is possible to prevent the diameter of the small aperture fromexpanding unexpectedly if there is a sudden increase in current whilethe laser is being used in practice.

If the light-emitting element is further provided with a secondwavefront converter for focussing light that is emitted from thelight-emitting portion and supplying the same to the first wavefrontconverter, it is possible to further improve the evanescent output.

In this case, it is preferable that the second wavefront converter isany one of a concave reflective mirror, a Fresnel lens, a waveguidelayer having a non-uniform spatial distribution of refractive indices, aplanar reflective mirror, or a convex lens, for focusing light emittedfrom the light-emitting portion onto the small aperture.

In addition, the first wavefront converter could have a non-uniformspatial distribution of refractive indices, with the radiating range oflight emitted from the light-emitting portion being output as outputlight after being converted into a radiating range that is smaller thanthe wavelength thereof, by a lens effect created by the spatialdistribution of refractive indices.

In this case, the spatial distribution of refractive indices is createdby varying effective refractive indices in a spatial manner, inaccordance with a plasma effect achieved by varying the injectiondensity of carriers in a spatial manner by adjusting values ofresistivity within the first waveform converter.

This invention also relates to a surface-emitting type of light-emittingelement comprising a multi-layered structure comprising a light-emittinglayer; and a pair of electrodes for supplying a current to thelight-emitting layer; wherein output light is output from alight-emitting surface of the multi-layered structure; and the pair ofelectrodes are provided in a recessed position from the light-emittingsurface toward the light-emitting layer side. This makes it possible tobring the light-emitting surface extremely close to an object to beilluminated.

If the light-emitting surface is a surface of a wavefront converterhaving a small aperture of a diameter that is smaller than thewavelength of light emitted from the light-emitting layer; and theoutput light comprises an evanescent wave that is output to the exteriorthrough this small aperture, it is possible to shine an evanescent wavereliably onto an object to be illuminated, by positioning the outputsurface of the evanescent wave sufficiently close to the target, such asan optical disc.

In this case, both of the pair of electrodes are provided on the sameside, on either a top surface side or a rear surface side of themulti-layered structure.

The configuration could be such that one of the pair of electrodes andthe light-emitting surface is provided on a main-surface side of themulti-layered structure and the other of the pair of electrodes isprovided on a rear-surface side of the multi-layered structure, or theconfiguration could be such that the light-emitting surface is providedon the main-surface side of the multi-layered structure and both of theelectrodes are provided on the rear-surface side of the multi-layeredstructure.

When the pair of electrodes are connected electrically to wires, if eachof the electrodes is provided in such a manner as to not protrude on theside from which emitted light is extracted, it is possible to bring thelight-emitting element sufficiently close to an object to beilluminated, to illuminate the object reliably with an evanescent wave.

It is also possible to improve the efficiency with which light isextracted, by further providing a transparent dielectric layer that isdisposed on a light-emitting surface of the small aperture.

It is further possible to prevent the small aperture from expandingunexpectedly if there is a sudden increase in current while the laser isbeing used in practice, by providing a transparent protective film thatis coated onto an inner wall of the small aperture.

If the light-emitting element is further provided with a secondwavefront converter for focusing light that is emitted from thelight-emitting layer and supplying the same to the small aperture, it ispossible to improve the light output even further.

In this case, the second wavefront converter is preferably any one of aconcave reflective mirror, a Fresnel lens, a waveguide layer having anon-uniform spatial distribution of refractive indices, a planarreflective mirror, or a convex lens, for focusing light emitted from thelight-emitting portion onto the small aperture.

A method of fabricating a light-emitting element in accordance with thepresent invention, wherein the light-emitting element has amulti-layered structure comprising a light-emitting portion made of anitride semiconductor, and a thin film in which is formed a smallaperture having a diameter that is smaller than the wavelength of lightemitted from the light-emitting portion; such that at least part of thelight emitted from the light-emitting portion is produced as anevanescent wave through the small aperture, comprises the steps of:forming the multi-layered structure; forming the thin film on a surfaceof the multi-layered structure; and opening up the small aperture in thethin film in a self-aligning manner, by supplying a current to thelight-emitting portion and illuminating light that is emitted from thelight-emitting portion onto the thin film. This makes it possible toopen up the small aperture, in an extremely easy and reliable manner,and also makes it unnecessary to use expensive equipment such as an FIB.

In this case, if the diameter of the small aperture is adjusted in thestep of opening up the small aperture, by monitoring light that isprojected through the small aperture with a detector, it is possible tocontrol the diameter of the aperture easily and reliably.

Furthermore, if the diameter of the small aperture is adjusted in thestep of opening up the small aperture, by monitoring light that isprojected through the small aperture with a detector, it is unnecessaryto place the detector too close to the light-emitting element.

This fabrication method could further comprise a step of coating aninner wall of the small aperture with a material that is transparentwith respect to light that is emitted from the light-emitting portion,after the step of opening up the small aperture. This makes it possibleto prevent the aperture from expanding unexpectedly if there is a suddenincrease in current while the laser is being used in practice.

The effects achieved by the above described configurations are discussedbelow.

First of all, the present invention makes it possible to implementsuitable beam characteristics, by providing a light-emitting element,which is made of a nitride semiconductor, and a wavefront converter.

In other words, it is possible to focus light from the light-emittingportion without using any form of optical system such as a lens, byproducing an evanescent wave through the small aperture. As a result,the spot size of the thus obtained evanescent wave can be made no morethan one-tenth the size of that in a conventional DVD system. This meansthat it is possible to implement an ultra-high-density optical discsystem or a magneto-optical disc system that has a recording capacitythat is at least one hundred times that of a conventional DVD system.

It is also unnecessary to adjust the lens to cope with changes in thewavelength, or adjust the optical axis within the pickup.

As described above, the present invention has many advantages from theindustrial point of view in that it provides an ultra-high-densityoptical disc system that is inexpensive and highly reliable, byimplementing a light-emitting element that has an extremely small spotsize.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detaileddescription given herebelow and from the accompanying drawings ofpreferred embodiments of the invention. However, the drawings are notintended to imply any limitation of the invention to a specificembodiment, but are for explanation and understanding only.

In these drawings:

FIG. 1 shows schematic views of a light-emitting element in accordancewith an embodiment of this invention, where FIG. 1A is a plan view ofthe light-emitting element and FIG. 1B is a section taken along the lineA-A′;

FIG. 2 is a graph showing how the light absorption coefficient ofsilicon depends upon wavelength;

FIGS. 3A to 3D are schematic views of a method of forming a smallaperture in accordance with this invention;

FIG. 4A is a schematic view of essential components of a light-emittingelement in accordance with a second embodiment of the invention;

FIG. 4B is a schematic view of a first variation of this secondembodiment;

FIG. 5A is a schematic view of a second variation of this secondembodiment;

FIG. 5B a schematic view of a third variation of this second embodiment;

FIG. 6 is a perspective view of a section through the structure of alight-emitting element in accordance with this third embodiment;

FIG. 7 is a schematic section through the structure of a light-emittingelement in accordance with a fourth embodiment of the invention;

FIG. 8 shows schematic views of the structure of a light-emittingelement in accordance with a fifth embodiment of the invention, withFIG. 8A being a section therethrough and FIG. 8B showing a base viewthereof;

FIG. 9 shows schematic views of the structure of a light-emittingelement in accordance with a sixth embodiment of the invention, withFIG. 9A being a section therethrough and FIG. 9B showing the planarpattern thereof;

FIG. 10 shows schematic views of the structure of a light-emittingelement in accordance with a seventh embodiment of the invention, withFIG. 10A being a transparent plan view thereof, FIG. 10B being a sectiontaken along the line A-A′, and FIG. 10C being a graph of thedistribution of resistivity and refractive index along the line A-A′ ofthe end portion of the waveguide layer;

FIGS. 11A to 11C are plan views illustrating states in which guidedlight is refracted by a wavefront converter 722;

FIG. 12 shows schematic views of the structure of a light-emittingelement in accordance with an eighth embodiment of the invention, withFIG. 12A being a transparent plan view thereof, FIG. 12B being a sectiontaken along the line A-A′, and FIG. 12C being a graph of thedistribution of resistivity and refractive index along the line A-A′ ofthe end portion of the waveguide layer;

FIG. 13 is a schematic section through the structure of a light-emittingelement in accordance with a ninth embodiment; and

FIG. 14 is a schematic section through a variation of the ninthembodiment of this invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

This invention makes it possible to produce an evanescent wave by usinga wavefront converter of a simple structure, and also makes it possibleto utilize spot-sized beam of light that is far smaller than that of theprior art. As a result, it is possible to implement anultra-high-density optical disc system that is 100 times as high as aDVD system of the prior art.

Embodiments of the present invention are described below with referenceto the accompanying drawings.

A schematic view of a light-emitting element in accordance with anembodiment of this invention is shown in FIG. 1. FIG. 1A is a plan viewthereof and FIG. 1B is a section taken along the line A-A′ in FIG. 1A. Alight-emitting element 100 shown in these figures is made of nitridesemiconductors and is a semiconductor laser that is known as anedge-emitting semiconductor laser. A wavefront converter 150 is providedon one facet thereof, in such a manner that evanescent waves can beextracted from a small aperture h thereof.

The shape of the small aperture h could be circular, or it could equallywell be any shape other than circular. In this document, “smallaperture” is defined as an aperture wherein the minimum distance ofspacings between edges facing the aperture is approximately the same asthe wavelength of light emitted by the light-emitting element, or evensmaller. If the small aperture h is a pin-hole, by way of example, thediameter thereof is approximately the same as the wavelength of thelight emitted thereby, or even smaller.

In FIG. 1A, the longitudinal direction indicated by the double-headedarrow is the direction of the resonator of the laser. Reference number150 denotes the wavefront converter that is the main focus of thisinvention. In this specific example, this wavefront converter 150 isformed of silicon (Si) and the small aperture h is opened in a portionthereof on which the laser light is projected, to dimensions that areapproximately the same as the wavelength of the laser light, or evensmaller. In addition, reference number 151 in FIG. 1A denotes adielectric film that acts as a spacer between the wavefront converterand the laser facet. In this specific example, the thickness thereof isdetermined by a silicon oxide (SiO₂) film of a thickness that isapproximately the same as the wavelength of the laser, or even smaller.A highly reflective film 160 that has a high coefficient of opticalreflectivity is provided on the other facet of the element.

The element of this configuration makes it possible to utilize anevanescent wave. In other words, if the diameter of the small aperture hthat is provided in the wavefront converter 150 is approximately thesame as the wavelength of light emitted from the laser, or even smaller,a propagated wave of the laser light is not projected outside throughthe small aperture h, and only an evanescent wave that diminishesdramatically with distance from the wavefront converter 150 isprojected. The size of the field of this evanescent wave isapproximately the same as the diameter of the small aperture h. That isto say, it is possible to obtain an extremely small spot-sized beam oflight that is approximately the same size as the diameter of the smallaperture h.

Use of this evanescent wave makes it possible to focus light to a sizethat is substantially the same as the diameter of the small aperture h,without using a lens. Thus, if the small aperture h is disposed in theclose vicinity of an optical disc (not shown in the figures), it ispossible to either detect or form tiny pits (indentations formed in thedisc surface) that are smaller than the wavelength of the laser light,making it possible to implement an ultra-high-density optical discsystem.

More specifically, in a method that uses an optical system implementedby a conventional lens, the diameter D of the beam spot of focused lightis given by: D=λ/NA, where λ is the wavelength of the light and NA isthe numerical aperture of the lens. Taking the rating for a prior-artdigital versatile disc (DVD) system by way of example, the wavelength ofthe light is 650 nm and the standard numerical aperture of the lens isapproximately 0.6. It is therefore clear that the diameter D of the beamspot of focused light is approximately 1 μm.

In contrast thereto, the present invention makes it possible to obtainan evanescent wave of a size that is substantially the same as that ofthe diameter of the small aperture h. If, for example, the diameter ofthe small aperture h is made to be 100 nm, the beam spot of theevanescent wave is roughly 100 nm or less.

The minimum size of pits formed on an optical disc is proportional tothe diameter of the beam spot of light. In other words, the recordingcapacity of an optical disc is inversely proportional to the square ofthe diameter of the beam spot of light. Thus the present invention makesit possible to increase the recording capacity of an optical disc to atleast one hundred times that of a conventional DVD system, even if thediameter of the small aperture h is 100 nm. Moreover, since there is noneed for an external optical system comprising components such as alens, it is possible to implement an optical pickup that does notrequire fine positioning with respect to the optical axis, has a lowerconstruction cost, and is highly reliable. In this case, thelight-emitting element in accordance with this invention is disposed inthe vicinity of an optical disc and means such as a piezoelectricelement could be used as an actuator, to implement the reading andwriting of data by an evanescent wave.

The description now turns to a specific example of the configuration ofthe semiconductor laser of this embodiment, with reference to FIG. 1B.The semiconductor laser is provided with a sapphire substrate 101,wherein the c surface is the main surface, and a multi-layer stack ofnitride semiconductors formed thereon. One example of such multi-layerstack is, starting from the substrate side: a 50-nm GaN buffer layer102; a high-temperature GaN buffer layer 103 (undoped, 0.5 μm thick,with a carrier density of 1×10¹⁷ cm⁻³); an n-type GaN contact layer 104(silicon-doped, 0.4 μm thick, with a carrier density of 2×10¹⁸ cm⁻³); ann-type AlGaN cladding layer 105 (8% aluminum composition, silicon-doped,0.5 μm thick, with a carrier density of 1×10¹⁸ cm⁻³); an n-type GaNguide layer 106 (silicon-doped, 0.3 μm thick, with a carrier density of1×10¹⁷ cm⁻³); an active layer 107 of an InGaN/InGaN multiple quantumwell (MQW) configuration (five pairs, with the well layers having an 18%indium composition and a thickness of 4 nm, and the barrier layershaving an 8% indium composition and a thickness of 2 nm); a p-type GaNguide layer 108 (magnesium-doped, 0.3 μm thick, with a carrier densityof 1×10¹⁷ cm⁻³); and a p-type AlGaN cladding layer 109 (8% aluminumcomposition, magnesium-doped, 0.5 μm thick, with a carrier density of2×10¹⁷ cm⁻³)

In addition, an n-type InGaN current-blocking layer 110 (silicon-doped,15% indium composition, 0.2 μm thick, with a carrier density of 1×10¹⁸cm⁻³) having a stripe-shaped hole of a width of 5 μm is formed on top ofthe p-type AlGaN cladding layer 109, and on top of that are formed ap-type GaN contact layer 111 (magnesium-doped, 0.3 μm thick, with acarrier density of 1×10¹⁷ cm⁻³) and a highly doped p-type GaN contactlayer 112 (magnesium-doped, 0.1 μm thick, with a carrier density of5×10¹⁷ cm⁻³).

Electrodes for supplying a current to the above laser structure could beelectrodes 140 of multiple layers of titanium (Ti) and gold (Au), forthe n-type contact layer 104, and an electrode 130 of multiple layers ofplatinum (Pt), titanium (Ti), platinum (Pt), and gold (Au), in sequencefrom the semiconductor side, for the p-type contact layer 112.

The method of fabricating this laser element will now be brieflydescribed. The multi-layered structure of nitride semiconductors couldbe formed by a method such as metal-organic chemical vapor deposition(MOCVD) or molecular beam epitaxy (MBE). Examples of source materialsthat can be used with MOCVD include trimethyl gallium, trimethylaluminum, trimethyl indium, and ammonia.

The method of exposing part of the n-type contact layer 104 for theformation of the n-side electrodes 140 could be a method of formation byetching after the entire multi-layered structure has been grown, or byselective growth after the n-type contact layer 104 has been formed.

After facets have been formed on the thus-constructed laser element by amethod such as cleavage, the highly reflective film 160 is formed on therearward facet and the dielectric film 151 and wavefront converter 150are formed on the forward facet. A so-called DBR (distributed Braggreflector) mirror, made up of a stack of a plurality of thin dielectricfilms, could be used as the highly reflective film 160, by way ofexample. A dielectric film of silicon oxide or silicon nitride could beused as the spacer 151, by way of example. The role of the spacer 151is, first of all, to maintain the optical reflectivity that is necessaryfor the laser surface. Thus a DBR mirror made up of a stack ofdielectric layers could also be used as the spacer 151. The spacer 151also has the role of improving the adhesive strength of the wavefrontconverter 150. It is therefore preferable to use a material thatprovides good adhesive strength between the facet of the laser elementand the wavefront converter 150. The spacer 151 has a further role ofproviding electrical insulation between the wavefront converter 150 andthe facet of the laser element. In other words, the side surfaces of then-type semiconductor layers and the p-type semiconductor layers areexposed on the facet of the laser element, as should be clear from FIG.1B, so that if a dielectric material is used as the material of thewavefront converter 150, it is necessary to cover the facet with aninsulator to ensure that the p-n junctions are not short-circuited. Fromthis viewpoint, it is preferable that the spacer 151 is constructed of amaterial that is insulating. Other than silicon oxide or siliconnitride, it is preferable that a dielectric film of a material such astitanium oxide (TiO₂) or aluminum oxide (Al₂O₃) is used as the materialof the spacer 151.

Another characteristic of the present invention is the use of silicon asthe material of the wavefront converter 150.

A graph of how the light absorption coefficient of silicon varies withwavelength is shown in FIG. 2. As shown in this figure, silicon istransparent to long wavelengths but its absorption coefficient of lightis extremely large in the short-wavelength region from blue onward. Theabsorption coefficient has a large value of 2×10⁵ cm⁻¹ for light ofwavelength 400 nm, for example. In other words, there is an extremelylarge absorption coefficient in the wavelength region of a nitridesemiconductor laser element. Thus if the film thickness thereof is onthe order of 200 nm, the laser light is substantially blocked and it istherefore possible to cause an evanescent wave to be generated from thesmall aperture alone.

In this case, the intensity of the evanescent wave decreasesexponentially from the edge portion on the inlet side of the apertureportion of the small aperture, and is dependent only on the range of thediameter of the small aperture. If the diameter of the small aperture his 0.1 μm, for example, the dependent range of the evanescent wave doesnot exceed approximately 0.1 μm. It is therefore important to ensurethat the wavefront converter 150 is sufficiently thin enough to ensurethat evanescent wave is produced outward at a high intensity. If siliconis used, as in this embodiment, it is possible to make the wavefrontconverter 150 thin enough at approximately 200 nm, so that theevanescent wave is produced outward at a large intensity. In addition tosilicon, further examples of the material of the film that configuresthe wavefront converter 150 which could be cited include a semiconductorwith a narrow energy gap such as geranium (Ge) or indium nitride (InN),a conductor such as carbon (C), or a metal such as indium (In), gold(Au), aluminum (Al), platinum (Pt), or silver (Ag). If one of thesemetal materials is used, the film could be made even thinner.

These materials could be deposited to a predetermined thickness onto thefacet of the laser element by various different methods such as vapordeposition, sputtering, or CVD.

One method of forming the small aperture h in the wavefront converter150 is to rapidly irradiate a laser beam of an output higher than thatof the laser element under ordinary usage conditions, to form the smallaperture.

Steps in a method of forming the small aperture in a self-aligningmanner in accordance with this invention are shown in FIGS. 3A to 3D. Inother words, FIGS. 3A to 3D illustrate the method of opening the smallaperture h in the laser shown in FIG. 1A, where portions that are thesame as those in FIG. 1A are given the same reference numbers andfurther description thereof is omitted.

First of all, a thin film 150A of silicon or the like is coated over thefacet of the laser, as shown in FIG. 3A. This thin film 150A will becomethe wavefront converter 150. A light detector 2000 is disposed facingthis facet. As will be described later in more detail, this lightdetector 2000 need only detect a propagating wave from the laser, not anevanescent wave emitted therefrom. It is therefore not necessary to makethe distance from the thin film 150A to the light detector 2000extremely short; this distance could be several millimeters to severalcentimeters. In this state, a current is injected through the laser togenerate a laser beam.

As the output of the laser beam increases until it reaches a certainoutput level, the thin film 150A is opened out in a self-aligning mannerby the laser beam, as shown in FIG. 3B. Ordinarily, a laser beamprojected from the facet of a laser has a spatial intensity distributionthat approximates to a Gaussian distribution. The size of the smallaperture that is opened in the thin film 150A can therefore be adjustedby controlling the output level and projection time of the laser beam asappropriate.

Once the aperture has been formed, the propagating light components ofthe laser are detected by the light detector 2000. If the relationshipsbetween the operating conditions of the laser, the aperture diameter,and the detection output of the light detector 2000 have beeninvestigated beforehand, therefore, it is possible to verify that apredetermined aperture has been created by monitoring the detectionoutput of the light detector 2000.

When the predetermined aperture has been created, the surface of thethin film 150A and the inner wall of the aperture are coated with aprotective film 150B, as shown in FIG. 3C. This protective film 150Bcould be formed of various different materials, such as silicon oxide(SiO₂) or silicon nitride (SiN). The protective film 150B has the roleof protecting the small aperture h when the laser (light-emittingelement) 100 is used subsequently under ordinary operating conditions.It has the role of preventing the diameter of the small aperture h fromexpanding unexpectedly if there is a sudden increase in current whilethe laser 100 is being used in practice, by way of example.

It is also possible to employ this protective film 150B as one part ofan anti-reflection (AR) film. In other words, it is preferable that anAR coating is formed on the opening portion of the small aperture h inorder to increase the efficiency with which the laser light isextracted. In accordance with this invention, the underlying dielectricfilm 151 and the protective film 150B can be used in their entirety asan AR coating. More specifically, if the wavelength of the laser lightis λ, the optical thickness of these layers could be λ/4. To ensurethis, it is possible to ensure that the optical thickness of theunderlying dielectric film 151 is λ/8 and the optical thickness of theprotective film 150B is λ/8.

Depending on the method used or the conditions of forming thisprotective film 150B, the protective film 150B could be formed to fillthe small aperture h, as shown in FIG. 3D. In such a case, the functionof the wavefront converter 150 is maintained because the protective film150B is transparent with respect to the wavelength of the laser. In theexample shown in FIG. 3D, if the optical thickness of the underlyingdielectric film 151 is λ/4 and the optical thickness within the smallaperture h of the protective film 150B is λ/2, the total opticalthickness is 3λ/4, which enables the combination to function as an ARcoating in the same manner as if the thickness is λ/4.

In the method in accordance with this invention, the laser output levelthat is necessary for forming the aperture can be determined asappropriate by factors such as the material and thickness of the thinfilm 150A. However this is implemented, the laser output for forming theaperture is greater than the output used during ordinary operatingconditions.

If, for example, a film of gold (Au) or silver (Ag) of a thickness ofapproximately 100 nm is used as the thin film 150A, when laser light hasa wavelength of approximately 400 nm, this film will exhibit sufficientlight-blocking characteristics under ordinary operating conditions toenable it to function as a wavefront converter. If a gold thin film 150Aof approximately this thickness is used, the aperture, that is, thesmall aperture h, can be formed reliably by setting the laser output togreater than normal.

If silicon (Si) is used as the material of the wavefront converter 150,for laser light that has a wavelength on the order of 400 nm, sufficientlight-blocking characteristics can be obtained by ensuring that thethickness of this film is approximately 200 nm, as can be understoodfrom FIG. 2. Even with the silicon thin film 150A, it is possible toopen up the small aperture h reliably in a self-aligning manner,provided the laser output is set to be greater than that during ordinaryuse.

Similarly, if the laser light has a wavelength on the order of 650 nm,as will be described later, use of a gold (Au) film of a thickness ofapproximately 100 nm as the thin film 150A enables it to act as thewavefront converter and, at the same time, makes it possible to open upthe small aperture h in a self-aligning fashion in accordance with thisinvention.

In the prior art, it has been necessary to employ etching using afocused ion beam (FIB) or the like, in order to open up the smallaperture h. However, it is not easy with such methods to align theposition of the small aperture h with the optical axis of the laserbeam, making it difficult to avoid insufficiencies in the outputcharacteristics due to factors such as positional displacements.

In contrast thereto, the method in accordance with this invention makesit possible to open up a self-aligned small aperture, in an extremelyeasy and reliable manner, and also makes it unnecessary to use expensiveequipment such as an FIB.

The description now turns to a second embodiment of the presentinvention.

A schematic view of essential components of a light-emitting element inaccordance with this second embodiment of the invention is shown in FIG.4A. This figure shows a section through the essential components, withthe front surface being a cross-sectional plane. A light-emittingelement 200A shown herein is based on a laser that is known as avertical-cavity type of surface-emitting laser, with a wavefrontconverter 225 being provided on the light-emitting surface thereof.

To describe the configuration of this element: a GaN buffer layer 211, aGaN/GaAlN multi-layer film 212, an n-type GaN contact layer 213, ann-type GaAlN cladding layer 214, an n-type GaN waveguide layer 215, anInGaN MQW active layer 216, a p-type GaN waveguide layer 217, a p-typeGaAlN cladding layer 218, and an n-type GaAlN current-confining layer219 are grown in that order on a sapphire substrate 210. The activelayer 216 could have an MQW structure comprising a stack of alternatingIn_(x)Ga_(1−x)N well layers and In_(y)Ga_(1−y)N barrier layers (wherex≧y). The crystal growth thereof could be achieved by a method such asMOCVD or MBE.

The n-type GaAlN current-confining layer 219 is partially etched away sothat it has a circular aperture portion, then a p-type GaN contact layer220 and a GaN/GaAlN multi-layer film 221 are formed thereon. The crystalgrowth thereof could be achieved by a method such as MOCVD or MBE.

This multi-layered structure is machined to remove two sides and leave amesa shape, as shown in the figures. Parts of each of the substrate 210and the n-type GaN contact layer 213 are exposed on either side of themesa. An SiO₂ film 222 is formed over the side surfaces of this mesaportion. A p-side electrode 223 is formed on top of the exposed portionof the substrate 210 and the SiO₂ film 222, and an n-side electrode 224is formed on top of the exposed portion of the n-type GaN contact layer213.

The wavefront converter 225 of a material such as silicon (Si) is thenformed on top of the GaN/GaAlN multi-layer film 221. A small aperture hof a diameter of approximately 100 nm is formed in the center of thiswavefront converter 225.

In the light-emitting element 200A shown in the figures, each of theGaN/GaAlN multi-layer film 212 and the GaN/GaAlN multi-layer film 221acts as a resonator mirror, forming a vertical resonator, so that thelight-emitting element has the configuration of a laser that is known asa vertical-cavity surface-emitting laser.

The wavefront converter 225 provided on the facet of the resonator hasthe function of converting light from within the resonator into anevanescent wave. In other words, the diameter of the small aperture h isapproximately 100 nm, in contrast to the wavelength of light emittedfrom the active layer 216 which is on the order of approximately 400 to500 nm, so it is less than the lasing wavelength of the laser element.For that reason, the laser beam is not projected to the exterior of thesmall aperture h as propagating light; it becomes an evanescent wavethat diminishes rapidly with distance from the small aperture h. It istherefore possible to obtain a spot-sized light beam that isapproximately the same size as the diameter of the small aperture h, aspreviously described with reference to the first embodiment of thisinvention.

In this embodiment too, it is possible to make the thickness of thewavefront converter 225 suitably thin by using silicon therefor, aspreviously described with reference to FIG. 2, so that an evanescentwave of a high intensity can be obtained from the small aperture h.

In this embodiment too, it is possible to open up the small aperture hin a self-aligning manner, by increasing the laser output to greaterthan that under ordinary conditions, as previously described withreference to FIG. 3.

In addition, the light-emitting surface of the surface-emitting laser inaccordance with this embodiment of the invention is provided withstepped surfaces to form a mesa, and the p-side electrode 223 is formedto extend as far as a base surface B of the corresponding step. Thep-side electrode 223 and the n-side electrode 224 are each connected towires W on the respective step surfaces. Moving the wire-bondingpositions away from the light-emitting surface of the element in thismanner ensures that the wires W do not interfere with a target such asan optical disc (not shown in the figure) and thus the wavefrontconverter 225 can be disposed sufficiently close thereto. Since thisinvention makes use of an evanescent wave that exists only within anextremely limited field, it is particularly important to have aconfiguration that makes it possible to position the wavefront converter225 sufficiently close to the target.

A perspective view of a section through the components of a firstvariation of the light-emitting element in accordance with thisembodiment is shown in FIG. 4B. Portions in this figure that are thesame as those described previously with reference to FIG. 4A are giventhe same reference numbers and further description thereof is omitted.In a light-emitting element 200B shown in FIG. 4B, each of the GaN/GaAlNmulti-layer film 212 and the GaN/GaAlN multi-layer film 221 acts as aresonator mirror, forming a vertical resonator, so that thelight-emitting element has the configuration of a laser that is known asa vertical-cavity surface-emitting laser. It should be noted, however,that a structure known as a λ-cavity is formed of the layers 214 to 218between the multi-layer films 212 and 221. In other words, it ispossible to have a configuration that is extremely effective ingenerating resonance of light in the vertical direction, by making theoverall optical thickness of the layers 214 to 218 substantially thesame as the wavelength λ of the laser beam.

In addition, the current-confining layer 219 in the light-emittingelement 200B is formed within the multi-layer film 221. Thiscurrent-confining layer 219 can be formed by selectively implantingprotons into the multi-layer film 221. If the multi-layer film 221comprises a layer of a material containing aluminum, such as AlGaN orAlN, the current-confining layer 219 could also be formed by selectivelyoxidizing this layer containing aluminum.

An optical thin film 270 of a dielectric material such as SiO₂ or SiN isformed between the multi-layer film 221 and the wavefront converter 225.If the optical thickness of this thin film 270 is such that it is λ/4with respect to the wavelength λ of the laser (an AR coating), it ispossible to improve the efficiency at which laser light is produced fromthe small aperture h and, at the same time, improve the reflectiveefficiency of laser light on the lower side of the wavefront converter225. Note that it is preferable to make this thin film 270 integral withthe protective film that forms part of the wavefront converter 225, andadjust the optical thickness thereof to be λ/4, as previously describedwith reference to FIG. 3C.

A perspective view of a section through the components of a secondvariation of the light-emitting element in accordance with thisembodiment is shown in FIG. 5A. A light-emitting element 200C shown inthis figure is also based on a vertical-cavity surface-emitting laser.The basic components thereof can be assumed to be identical to thoseillustrated in FIGS. 4A and 4B, so portions having the same functionsare given the same reference numbers and further description thereof isomitted. This variation differs from the light-emitting elements shownin FIGS. 4A and 4B in that electrodes are provided on upper and lowersurfaces of the light-emitting element, by removing the sapphiresubstrate 210 after the crystalline growth steps.

In order to implement this configuration, it is necessary to provide aGaN layer 250 of a thickness of approximately 80 μm between themulti-layer film 212 and the buffer layer 211 grown on a substrate 210.The buffer layer 211 and the substrate 210 are not shown in FIG. 5A.Since the layer 250 acts as a contact layer from the rear surface, it ispreferably n-type with a carrier density of 10¹⁸/cm³. To remove thesubstrate 210, it is necessary to employ some means such as using amaterial for the buffer layer 211 that can easily be etched, such asInN, or utilizing an SiO₂ film that is formed locally. In addition, themulti-layer film 212 need not be formed during the crystal growth steps;instead, the n-side electrode could be made transparent and a reflectivefilm could be formed on the outer side of this electrode 224.

In this variation of the embodiment too, it is possible to open up thesmall aperture h in a self-aligning manner, by increasing the laseroutput to greater than that under ordinary conditions, as previouslydescribed with reference to FIG. 3.

A perspective view of a section through the components of a thirdvariation of the light-emitting element in accordance with thisembodiment is shown in FIG. 5B. A light-emitting element 200D shown inthis figure is also based on a vertical-cavity surface-emitting laser.The basic components thereof can be assumed to be identical to thoseillustrated in FIGS. 4A to 5A, so portions having the same functions aregiven the same reference numbers and further description thereof isomitted. This variation is structurally similar to that shown in FIG.5A, in that the electrodes are provided on surfaces above and below thelight-emitting element, by using a conductive GaAs substrate as thesubstrate 250.

It should be noted, however, that the multi-layered structure and thematerials of the element are different. In other words, thelight-emitting element 200D is configured of an n-type GaAs substrate250, an n-type GaAs buffer layer 211, an n-type Ga_(0.5)Al_(0.5)As/AlAsmulti-layer film 212, an n-type InGaAlP cladding layer 214, an InGaAlPwaveguide layer 215, an MQW active layer 216 of InGaP/InGaAlP, anInGaAlP waveguide layer 217, a p-type InGaAlP cladding layer 218, ap-type Ga_(0.5)Al_(0.5)As/AlAs multi-layer film 221, and acurrent-confining layer 219.

The current-confining layer 219 can be formed by selective oxidation ofthe multi-layer film 221 or by selective implantation of protonsthereinto.

In this variation of the embodiment too, the layers from the claddinglayer 214 to the cladding layer 218 configure a λ-cavity, in the samemanner as that described previously with reference to FIG. 4B, and themulti-layer films 212 and 221 are provided above and below as reflectivemirrors.

The laser of this variation makes it possible to obtain a high-outputevanescent wave in the vicinity of a wavelength of 650 nm, by adjustingthe composition and structure of the active layer 216 as appropriate. Inorder to accommodate laser light in the vicinity of this wavelength of650 nm, it is preferable to use gold (Au) or silver (Ag), which havehigh absorption coefficients in that wavelength band, as the material ofthe wavefront converter 225.

In this variation of the embodiment too, it is possible to open up thesmall aperture h in a self-aligning manner, by increasing the laseroutput to greater than that under ordinary conditions, as previouslydescribed with reference to FIG. 3.

If the materials of the various layers in the structure of thelight-emitting element 200D are modified, it is possible to obtain anevanescent wave in the vicinity of a wavelength of 850 nm. In otherwords, if n-type Ga_(0.5)Al_(0.5)As/AlAs is used for the multi-layerfilm 212, n-type GaAlAs is used for the cladding layer 214, a MQWstructure of GaAs/GaAlAs is used for the active layer 216, p-type AlGaAsis used for the cladding layer 218, and p-type Ga_(0.5)Al_(0.5)As/AlAsis used for the multi-layer film 221, it is possible to obtain anevanescent wave in the vicinity of a wavelength of 850 nm. In this casetoo, the configuration could be such that the cladding layers 214 to 218form a λ-cavity.

Similarly, in this case too, the current-confining layer 219 can beformed by selective oxidation of the multi-layer film 221 or byselective implantation of protons thereinto.

The description now turns to a third embodiment of the presentinvention.

A perspective view of a section through the structure of alight-emitting element in accordance with this third embodiment is shownin FIG. 6. A light-emitting element 300 shown in this figure is alsobased on a vertical-cavity surface-emitting laser. The basic componentsthereof can be assumed to be identical to those of the light-emittingelements 200A to 200D that were described previously with reference tothe second embodiment, so portions having the same functions are giventhe same reference numbers and further description thereof is omitted.This embodiment is characterized in that a plurality of resonators andsmall apertures are formed therein, to create an array. If thislight-emitting element 300 is used as a pickup head, it is possible tosimultaneously record or reproduce a plurality of tracks of an opticaldisk (not shown in the figure).

Note that this embodiment is illustrated as having a one-dimensionalarray of resonators and small apertures h, but it should be obvious tothose skilled in the art that this invention is not limited thereto andthus a two-dimensional array is also possible within the scope of thisinvention. The structure of the surface-emitting laser shown in thisfigure, in particular, is convenient in that it makes it possible toimplement a two-dimensional array in a simple manner.

The description now turns to a fourth embodiment of the presentinvention.

A schematic section through the structure of a light-emitting element400 in accordance with this fourth embodiment of the invention is shownin FIG. 7. The light-emitting element 400 of this figure is also basedon a vertical-cavity surface-emitting laser. Reference number 450denotes a sapphire substrate, and a GaN buffer layer 451, an n-type GaNcontact layer 453, an n-type GaAlN cladding layer 454, an n-type GaNwaveguide layer 455, an InGaN MQW active layer 456, a p-type GaNwaveguide layer 457, a p-type GaAlN cladding layer 458, an n-type GaAlNcurrent-confining layer 459, and a p-type GaN contact layer 460 aregrown on top of this substrate 450. In addition, reference number 461denotes an SiO₂/TiO₂multi-layer film, reference number 462 denotes anSiO₂ film, reference number 463 denotes another SiO₂/TiO₂ multi-layerfilm, reference number 464 denotes a p-side electrode, reference number465 denotes an n-side electrode, and reference number 466 denotes asilicon film provided with a small aperture h.

This embodiment of the invention is characterized in having theSiO₂/TiO₂multi-layer film 463 that acts as a second wavefront converter,in addition to the silicon film 466 that acts as the first wavefrontconverter. This second wavefront converter is obtained by subjecting therear surface side of the sapphire substrate to curved-surface machiningthen forming the multi-layer film 463 on this surface. Since light isreturned to the multi-layer film mirror of the small aperture by thiscurved-surface mirror, a stable, highly efficient resonator is formedthereby.

The description now turns to a fifth embodiment of the presentinvention.

Schematic views of the structure of the light-emitting element inaccordance with this fifth embodiment of the invention are shown in FIG.8, with FIG. 8A being a section therethrough and FIG. 8B showing a baseview of an essential component thereof. A light-emitting element 500 ofthese figures is also based on a vertical-cavity surface-emitting laser.Reference number 570 denotes a sapphire substrate, and a GaN bufferlayer 571, an n-type GaN contact layer 573, an n-type GaAlN claddinglayer 574, an n-type GaN waveguide layer 575, an InGaN MQW active layer576, a p-type GaN waveguide layer 577, a p-type GaAlN cladding layer578, an n-type GaAlN current-confining layer 579, and a p-type GaNcontact layer 580 are grown on top of this substrate 570. In addition,reference number 581 denotes an SiO₂/TiO₂ multi-layer film, referencenumber 582 denotes an SiO₂ film, reference number 584 denotes a p-sideelectrode, reference number 585 denotes an n-side electrode, andreference number 586 denotes a silicon film provided with a smallaperture which acts as a first wavefront converter. Reference number 587denotes a diffraction grating lens that acts as a second wavefrontconverter. The diffraction grating lens 587 is a Fresnel lens which usesa material such as SiO₂ and which has a planar pattern as shown in FIG.8B.

In this embodiment of the invention, the SiO₂/TiO₂ multi-layer film 581and the diffraction grating lens 587 together configure a resonator.Since the light reflected back by the diffraction grating lens 587 isreturned to the multi-layer film 581 of the small aperture h, a stable,highly efficient resonator is formed thereby.

Projected light that has passed through the diffraction grating lens 587can also be focused on the lower side of the substrate 570. In otherwords, it is possible to produce an evanescent wave through the smallaperture h of the silicon film 586 and, at the same time, it is possibleto produce light that has been focused by the diffraction grating lens,from the rear-surface side of the substrate. This light on therear-surface side can be used, for example, as a monitoring light forcontrolling the laser output or as a probe light for detecting thelocation of a pickup of an optical disc system.

If only the light from the rear-surface side is to be used, the smallaperture h on the front-surface side is not necessary and thus it is notabsolutely necessary to form the silicon film 586. If light from therear-surface side is not to be used, on the other hand, the reflectivityof the diffraction grating lens 587 could be increased to make a highlyreflective type of diffraction grating lens.

The description now turns to a sixth embodiment of the presentinvention.

Schematic views of the structure of the light-emitting element inaccordance with this sixth embodiment of the invention are shown in FIG.9, with FIG. 9A being a section therethrough and FIG. 9B showing theplanar pattern of an essential component thereof. A light-emittingelement 600 of these figures is also based on a vertical-cavitysurface-emitting laser. Reference number 610 denotes a sapphiresubstrate, a first GaN buffer layer 611 is deposited upon this substrate610, then an SiO₂layer 612, a metal film 613, and another SiO₂ layer 614are further deposited thereon. These SiO₂ layers and the metal film 612to 614 are partially removed by etching to form a diffraction gratinglens 630 of the pattern shown in FIG. 9B. A second GaN buffer layer 615is grown on top of this structure. During the growth of this second GaNbuffer layer 615, epitaxial growth is generated from the lower first GaNbuffer layer 611 on the lower level, through the opening of thediffraction grating lens 630, and also lateral growth proceeds withinthe plane of the surface, to obtain the second GaN buffer layer 615 in amonocrystalline form.

An n-type GaN contact layer 616, an n-type GaAlN cladding layer 617, ann-type GaN waveguide layer 618, an InGaN MQW active layer 619, a p-typeGaN waveguide layer 620, a p-type GaAlN cladding layer 621, an n-typeGaAlN current-confining layer 622, a p-type GaN layer 623, and a p-typeGaN/p-type GaAlN multi-layer film 624 are then grown, in that order, ontop of the above structure. Reference number 625 denotes a p-sideelectrode, reference number 626 denotes an n-type electrode, referencenumber 627 denotes an SiO₂/TiO₂multi-layer film, and 628 denotes asilicon film provided with a small aperture h.

In this embodiment of the invention, the silicon film 628 acts as afirst wavefront converter and the diffraction grating lens 630 on top ofthe sapphire substrate acts as a second wavefront converter. The laserlight can be focused onto the rear surface of the sapphire substrate bythe diffraction grating lens 630. In addition, a very intense evanescentwave can be produced through the small aperture h of the silicon film628 that is the first wavefront converter.

Since the refractive index of the sapphire substrate 610 is greater than1 in this case, it is possible to make the numerical aperture (NA) ofthe diffraction grating lens 630 also greater than 1. In such a case,the field of light on the lower side of the substrate 610 is evanescentand a spot that is smaller than the wavelength can be obtained, even ifthe first wavefront converter, that is, the silicon film 628 providedwith the small aperture, is omitted. It should be obvious that it ispossible to add the small aperture, as described with respect to thisembodiment, to obtain an evanescent wave of an extremely small spotsize.

The description now turns to a seventh embodiment of the presentinvention.

Schematic views of the structure of the light-emitting element inaccordance with this seventh embodiment of the invention are shown inFIG. 10, with FIG. 10A being a transparent plan view of essentialcomponents thereof, FIG. 10B being a section taken along the line A-A′,and FIG. 10C being a graph of the distribution of resistivity andrefractive index along the line A-A′ of the end portion of the waveguidelayer.

A light-emitting element 700 of these figures is based on anedge-emitting laser. The description first concerns the structure of thesection shown in FIG. 10B, where reference number 710 denotes a sapphiresubstrate. On top of this substrate 710 are formed a GaN buffer layer711, an n-type GaN contact layer 712, an n-type GaAlN cladding layer713, an n-type GaN contact layer 712, an n-type GaAlN cladding layer713, an n-type GaN waveguide layer 714, an InGaN multi-layer MQW activelayer 715, a p-type GaN waveguide layer 716, a p-type GaAlN claddinglayer 717, an n-type GaAlN current-confining layer 718, and a p-type GaNcontact layer 719. Reference number 720 denotes a p-side electrode andreference number 721 denotes an n-side electrode.

In this embodiment of the invention, a wavefront converter 722 isprovided in the vicinity of a projection facet on one side of the laserelement. The wavefront converter 722 is configured from a localmodification of the resistivity of the p-type GaN waveguide layer 716,in a portion in the vicinity of the facet. In other words, theconfiguration is such that the resistivity in the center of this stripeis high whereas that at the outer sides thereof is low, as shown in FIG.10C. Since the implantation of carriers into the active layer ofportions outside the stripe is greater than that at the center of thestripe, this effectively lowers the refractive index on the outer sidesof the stripe, due to the plasma effect, as shown in FIG. 10C. Thewavefront converter therefore acts as a lens with respect to the guidedlight.

Plan views that illustrate the states in which guided light is refractedby the wavefront converter 722 are shown in FIG. 11. In other words, thedistribution of light within the wavefront converter 722 can be dividedinto a number of manifestations, depending on the refractive indexthereof and the length L of the wavefront converter 722.

In the example shown in FIG. 11A, the length L of the wavefrontconverter is greater than the focal length of the guided light. As aresult, the guided light comes to a focus outside the facet, as shown inthis figure. In the example shown in FIG. 11B, the focal length of theguided light matches the length L of the wavefront converter. As aresult, the guided light comes to a focus at the facet. In the exampleshown in FIG. 11C, the length L of the wavefront converter is twice thefocal length of the guided light. As a result, the light is projectedparallel from the facet of the laser, as shown in this figure.

In this manner, this embodiment of the invention makes it possible tocontrol the distribution of intensity of the projected light, byadjusting the refractive index distribution and the length L of thewavefront converter 722, to provide a desirable projected wavefront,even over a comparatively wide stripe.

The wavefront converter 722 can be fabricated by shining an electronbeam thereon, by way of example. In other words, after the p-type GaNwaveguide layer 716 has been formed, acceptors within the stripe portionare activated by irradiating an electron beam thereon. During this time,the distribution of this electron beam illumination is adjusted so thatthe dosage of electrons is larger in the outer portions of the stripebut lower in the central portion thereof. This makes it possible toimplement a structure having a low resistance on the outer sides and ahigh resistance in the center.

Note that since this method forms a stripe-shaped portion by electronbeam illumination, the n-type GaAlN current-confining layer 718 is notabsolutely necessary. In other words, if there is no electron beamillumination on the outer sides of the stripe, no current will flowtherethrough because the resistance there is higher, and thus the stripeitself can implement current constriction.

Another method of forming the wavefront converter 722 is ionimplantation. In other words, a p-type GaN waveguide layer 716 having adistribution of resistivities similar to that of FIG. 10C can be formedby implanting a p-type impurity such as magnesium (Mg) into the p-typeGaN waveguide layer 716 at a certain dosage distribution, then annealingthe element if necessary.

Alternatively, a p-type GaN waveguide layer 716 having a distribution ofresistivities similar to that of FIG. 10C can be fabricated by doping ap-type impurity uniformly during the crystal growth of the p-type GaNwaveguide layer 716, then implanting ions of hydrogen (H) at a certaindosage distribution. This makes use of a phenomenon such that the p-typeimpurity, such as magnesium, that is doped during the crystal growth isactivated by the hydrogen ion implantation.

Yet another method could be one in which both a p-type impurity such asmagnesium and an n-type impurity such as silicon are doped into thep-type GaN waveguide layer 716, where the density distribution of eitherone or both of these impurities is controlled. That is to say, a p-typeGaN waveguide layer 716 having a distribution of resistivities similarto that of FIG. 10C can be fabricated by varying the amount of dopedn-type impurity with respect to that of the p-type impurity to controlthe compensation of acceptors.

In this example, the resistivity of the p-type GaN waveguide layer 716is modified, but it should be obvious to those skilled in the art thatthe resistivity of the active layer 715 could also be modified.Furthermore, the resistivity of the p-type GaAlN cladding layer 717could also be modified in a similar manner, or the resistivities of boththe waveguide layer 716 and the cladding layer 717 could be modified.

The description now turns to an eighth embodiment of the presentinvention.

Schematic views of the structure of the light-emitting element inaccordance with this eighth embodiment of the invention are shown inFIG. 12, with FIG. 12A being a transparent plan view of essentialcomponents thereof, FIG. 12B being a section taken along the line A-A′,and FIG. 12C being a graph of the distribution of resistivity againstrefractive index along the line A-A′ of the end portion of the waveguidelayer. A light-emitting element 700 shown in these figures is providedwith a wavefront converter 830 having a small aperture h, on the faceton the light-emitting side of the light-emitting element 700 shown inFIG. 10.

In this case, details of the light-emitting element 800 such as thefacet structure thereof are the same as those described previously withreference to FIG. 10, so further description thereof is omitted.

In this embodiment of the invention, the guided light is focused by thewavefront converter 722 and can be extracted to the exterior as anevanescent wave through the small aperture h of the wavefront converter830. To increase the intensity of the evanescent wave that is focusedfrom the guided light in the small aperture h, it is preferable that therefractive index distribution and the length L of the wavefrontconverter 722 are set in such a manner that the guided light is focusedat the facet, as shown in FIG. 11B.

This embodiment of the invention is advantageous in that it makes itpossible to produce an evanescent wave of an extremely high intensity,by further focusing the guided light in an edge-emitting laser.

The description now turns to a ninth embodiment of the presentinvention.

A schematic section through the structure of a light-emitting element inaccordance with this ninth embodiment is shown in FIG. 13. This sectionis taken laterally in the direction of the resonator formed by alight-emitting layer. Other layers such as guide layers, claddinglayers, and contact layers that may be formed between a substrate 1101and a light-emitting layer 1116 or between a light-emitting layer 1116and an electrode 1120 omitted from this figure. This embodiment of theinvention is advantageous in that the output intensity can be furtherincreased, because a region in which the gain is high can be formed overa longer distance. The light generated in the light-emitting layer 1116resonates between a highly reflective film 1130 and a wavefrontconverter 1128 via a highly reflective film 1131, then can be extractedto the exterior by a small aperture h formed within the wavefrontconverter 1128. In this case, it is preferable that a dielectric filmhaving a multi-layered structure is formed as the highly reflective film1131 on top of a semiconductor surface that is inclined at an angle of45 degrees to the major plane of the substrate 1101.

A variation of this embodiment of the invention is shown in FIG. 14. Inthis figure too, other layers such as guide layers, cladding layers, andcontact layers that may be formed above and below a light-emitting layerare omitted. This embodiment differs from the light-emitting elementshown in FIG. 12 in having a configuration in which an absorbent layer1228 having a small aperture h is formed on top of a lens 1229 that ismachined from a substrate 1101. This variation of the invention makes itpossible to increase the intensity of the evanescent wave that isemitted through the small aperture h, by providing the lens 1229.

The present invention has been described above with reference tospecific examples thereof. However, the present invention is not limitedto these specific examples.

For example, the structural configurations of the edge-emitting lasersand surface-emitting lasers described above are nothing more thanexamples and thus other types of current injection structures,current-confining structures, and combinations of materials can be usedto similar effect.

In addition, the material used for the substrate is not limited tosapphire, and thus various effects can be achieved by using insulatingsubstrates of materials such as spinel, MgO, ScAlMgO₄, LaSrGaO₄, or(LaSr)(AlTa)O₃, or conductive substrates of materials such as SiC, Si,or GaN in a similar manner.

In addition to the previously described edge-emitting lasers andsurface-emitting lasers, the effects of the invention can be obtained byusing edge-emitting light-emitting diodes (LEDs) or surface-emittingLEDs in a similar manner.

Furthermore, the shape and size of the small aperture are not limited tothose described above, and similar effects can be obtained by setting asappropriate the relationship of those dimensions with respect to thewavelength of the light emitted from the light-emitting portion.

While the present invention has been disclosed in terms of preferredembodiments, in order to facilitate better understanding thereof, itshould be appreciated that the invention can be embodied in various wayswithout departing from the principle of the invention. Therefore, theinvention should be understood to include all possible embodiments andmodifications to the shown embodiments which can be implemented withoutdeparting from the principle of the invention as set forth in theappended claims.

What is claimed is:
 1. A light-emitting element of a surface-emittingtype, comprising: a substrate having a first surface and a secondsurface, said substrate being transparent to light with a wavelength ofλ; a diffraction grating lens overlying said first surface of saidsubstrate; a first conductive type semiconductor layer overlying saiddiffraction grating lens; a light emitting layer formed on said firstconductive type semiconductor layer and emitting light with a wavelengthof λ by current injection; a second conductive type semiconductor layerformed on said light-emitting layer and having an electrode formingsurface; a wavefront converter of a material opaque to the light fromsaid light-emitting layer, formed on said second surface of saidsubstrate, and having a small aperture with a diameter smaller than awavelength of the light, the light from said light-emitting layerpassing through said small aperture to be emitted as an output light,said output light including an evanescent wave; a first electrode formedon a surface of said first conductive type semiconductor layer on theside of said light-emitting layer, and electrically connected to saidfirst conductive type semiconductor layer; and a second electrode formedon said electrode forming surface of said second conductive typesemiconductor layer, said diffraction grating lens focusing lightemitted from said light-emitting layer on said small aperture of saidwavefront converter.
 2. The light-emitting element as defined in claim1, further comprising: a first buffer layer formed on said first surfaceof said substrate, said diffraction grating lens being formed on saidfirst buffer layer; and a second buffer layer formed on said diffractiongrating lens.
 3. The light-emitting element as defined in claim 2,wherein said first buffer layer, said second buffer layer, said firstconductive type semiconductor layer, said light-emitting layer, and saidsecond conductive type semiconductor layer are formed of a nitridesemiconductor.
 4. The light-emitting element as defined in claim 3,wherein said substrate is a sapphire substrate.
 5. The light-emittingelement as defined in clam 2, wherein said diffraction grating lenscontains SiO₂.
 6. The light-emitting element as defined in claim 1,wherein said light-emitting element is a vertical-cavitysurface-emitting laser.